1. Technical Field of the Invention
The present invention relates generally to a fuel injection system which may be employed with automotive internal combustion engines to sample a deviation of the quantity of fuel actually sprayed by a fuel injector from a target quantity to learn an injection characteristic of the fuel injector.
2. Background Art
There are known fuel injection systems for diesel engines which are designed to spray a small quantity of fuel into the engine (usually called a pilot injection) prior to a main injection of fuel in order to reduce combustion noise or NOx emissions. In order to emphasize the beneficial effects of the pilot injection, it is essential to improve the accuracy in controlling the quantity of fuel to be sprayed from the fuel injector. The fuel injection systems are, therefore, designed to sample a deviation of the quantity of fuel actually sprayed by the fuel injector (which will also be referred to as an actual injection quantity below) from a target quantity to correct a command injection duration (also called an injection period) for which the fuel injector is to be opened so as to minimize the deviation.
For example, Japanese Patent First Publication No. 2005-155360 proposes a fuel injection system for diesel engines which is engineered to perform an injection quantity learning operation to spray a single jet of fuel into one of cylinders of the engine when the engine is placed in a non-fuel injection condition wherein a drive pulse signal indicating that a target quantity of fuel to be sprayed from the fuel injector is smaller than zero (0) is outputted to the fuel injector, for example, when the engine is undergoing a fuel cut while the gear of the engine is being changed or the engine is decelerating and to sample a resulting change in speed of the engine to calculate the actual injection quantity. If the actual injection quantity is deviated from the target quantity, the fuel injection system corrects the injection duration (i.e., an on-duration) for which the fuel is to be opened so as to minimize a deviation.
The above fuel injection system is capable of using the actual injection quantity learned by a single jet of fuel sprayed from the fuel injector to correct a corresponding injection duration accurately. In other words, the accuracy is ensured in correcting the injection duration corresponding exactly to or around the quantity of the fuel sprayed in the injection quantity learning operation.
The fuel injection system is, however, lacking in the accuracy of correcting the injection duration to spray the quantity of fuel which is different from that sprayed in the injection quantity learning operation. Typical fuel injectors each have a correlation between the injection duration and the actual injection quantity. A mathematical line representing such a correlation has some inclination which is usually different between the fuel injectors and subjected to a change with the aging of the fuel injector. In such a case, the accuracy may be lacking in correcting the injection duration using the actual injection quantity, as calculated by spraying a single jet of fuel from the fuel injector. Additionally, when it is required to correct the injection duration which is different from that for which the fuel has been sprayed in the injection quantity learning operation, it may be lacking in accuracy.
When different quantities of fuel are sprayed in sequence to learn all actual injection quantities corresponding to a required number of injection durations, it will consume much time undesirably.